Bookworm: Install script may be overwriting the /boot/config.txt symlink with a file

[Gadgetoid]

Noticed here https://forums.raspberrypi.com/viewtopic.php?t=357956

And I also ran into this.

I think something in our installer script is resolving the symlink in its input stream, but then inadvertently outputting as if it were a file. We should probably detect /boot/firmware/config.txt and write there directly, with a fallback to /boot/config.txt if it doesn't exist.